Prior art methods for producing styrene are generally carried out by passing a mixture of ethylbenzene and steam over a fixed bed of dehydrogenation catalyst. In order to heat the reactants to reaction temperature, it is also general practice to admix the ethylbenzene, which is at a temperature significantly below reaction temperature, with steam which has been superheated to a temperature above the reaction temperature so that the mixture is at reaction temperature as it passes over the dehydrogenation catalyst. Since the basic chemical reaction involved, namely the dehydrogenation of ethylbenzene to styrene, is endothermic, there is a significant decrease in the reaction zone temperature as the reaction proceeds. It is not unusual in these prior art processes to witness a drop of perhaps 50.degree. C. to 100.degree. C. within the reaction zone. Naturally, as the temperature decreases, the rapidity of the reaction also decreases so that the overall efficiency of the process declines to a point where it would be economically unattractive unless processing means were found to overcome this disadvantage.
The prior art attempted to solve this problem by drastically increasing the temperature of the superheated steam so that the difference between the inlet temperature of the reactants and the outlet temperature of the reaction products averaged generally the required reaction temperature. However, it was noted that at the instant the superheated steam is admixed with the ethylbenzene, the ethylbenzene undergoes decomposition or cracking through the pyrolytic reaction. In many instances, such pyrolysis is effected to such a degree that the process becomes uneconomical due to the loss of ethylbenzene to carbon monoxide, carbon dioxide, polymeric materials, tars, etc. Another disadvantage is involved with the utility costs in raising the temperature of large quantities of steam to a level far above that required for effecting the dehydrogenation of the ethylbenzene. Additionally, in spite of all these efforts to control the reaction, the conversion of ethylbenzene to styrene remained at approximately the 30 to 40 percent conversion level.
More recently, the prior art has suggested means for increasing the level of conversion by utilizing various schemes for admixing ethylbenzene and steam in such a way as to avoid the pyrolytic reaction. One method has been to split the steam into several portions whereby additional steam is added between catalytic zones in order to reheat the reactants to reaction temperature. In these latter processes, conversions as high as 50% for ethylbenzene to styrene are alleged. However, these latter process schemes do not indicate the method by which the steam and ethylbenzene are heated with the result that utility costs are still prohibitively high for the achievement of the increased conversion level.
U.S. Pat. No. 3,402,212 discloses a method in which steam and hydrocarbons are heated to reaction temperature and maintained thereat in such a fashion that the conversion of ethylbenzene to styrene per pass exceeds 50 weight percent and requires no more than 5 pounds of steam per pound by styrene produced.
U.S. Pat. No. 3,330,878 discloses an improved process for the preparation of styrene from ethylbenzene using at least two dehydrogenation reactors with an intermediate indirect heating step.
U.S. Pat. No. 3,855,330 discloses a process which reportedly could lead to styrene yields of more than 70% in a single reactor by disposing a layer of an oxidation catalyst downstream of a dehydrogenation catalyst layer for combusting hydrogen formed in the reaction without affecting the hydrocarbons present. The oxidation catalyst layer is followed by a second layer of dehydrogenation catalyst.
However, these prior art processes do not indicate the method by which high temperature steam and a major fraction of a hydrocarbon feed mixture containing a minor amount of steam are introduced to the first stage of a multistage dehydrogenation process containing more than two stages and the remaining minor fraction of hydrocarbon feed mixture is admixed with the reaction product of the first stage and fed to the second stage of the process.
Accordingly, it is an object of this invention to provide for an endothermic vapor phase catalytic conversion of hydrocarbons in a multistage dehydrogenation process.
It is another object of this invention to provide a process for the dehydrogenation of ethylbenzene to styrene characterized by high conversion per pass of ethylbenzene to styrene, high purity of recovered styrene and low decline of ethylbenzene conversion over the course of a production run.
It is another object of this invention to effect a more economical method of dehydrogenating ethylbenzene to produce styrene in high conversion and purity.
It is a specific object of this invention to provide an improved multistage ethylbenzene dehydrogenation process in which steam and a major fraction of a hydrocarbon feed is fed to a first stage and a remaining fraction of the hydrocarbon feed and reaction product of the first stage are fed to a second stage.